1. Field of the Invention
The invention relates to an agent for preventing or eliminating foam, especially in aqueous systems, for which a defoaming agent is dissolved or dispersed in organic solvents or diluents, which optionally contain water, and which additionally may contain finely particulate, optionally hydrophobized, silica.
2. Description of the Prior Art
It is well known that polyoxyalkylene monools or diols or their ethers exhibit foam-preventing or foam-eliminating properties in aqueous systems when they are insoluble in these systems. As a rule, the monools or diols of polyaddition products of ethylene oxide and propylene oxide have been used for this purpose. Polyaddition products which contain oxyethylene units exclusively, develop their defoaming action at or after exceeding their cloud point in aqueous solution.
It is furthermore known that vegetable or mineral oils or silicone oils have defoaming activity. Copolymers of polyoxyalkylene and polysiloxane blocks have also been prepared and used to increase the defoaming properties. Especially preferred in this connection are block copolymers whose polyoxyalkylene block contains at least 80 weight percent of oxypropylene units. Such a preparation is described, for example, in German Auslegeschrift No. 24 43 853. The defoaming effectiveness of this preparation can be increased even further by the addition of highly dispersed silica which is prepared, for example, by flame hydrolysis.
German Pat. No. 23 45 335 describes a defoamer for aqueous solutions or dispersions which contains 80 to 95 weight percent of a mineral oil or vegetable or animal oil, 1 to 7.5 weight percent of highly dispersed silica or highly dispersed aluminum oxide, 1.0 to 10 weight percent of a methylpolysiloxane-polyoxyalkylene block copolymer, whose methylpolysiloxane block constitutes 10 to 60 weight percent of the polymer and whose polyoxyalkylene block has 80 to 100 weight percent of oxypropylene units.